simple infinitive; _to
speak_, or _speak_ + _to_ = the Anglo-Saxon _to spr['e]canne_, an
infinitive in the dative case.

s. 422. _Convertibility._--In the English language, the greater part of the
words may, as far as their form is concerned, be one part of speech as well
as another. Thus the combinations _s-a-n-th_, or _f-r-e-n-k_, if they
existed at all, might exist as either nouns or verbs, as either
substantives or adjectives, as conjunctions, adverbs, or prepositions. This
is not the case in the Greek languages. There, if a word be a substantive,
it will probably end in -s; if an infinitive verb, in -ein, &c. The
bearings of this difference between languages like the English and
languages like the Greek will soon appear.

At present, it is sufficient to say that a word, originally one part of
speech (e.g., a noun), may become another (e.g., a verb). This may be
called the convertibility of words.

There is an etymological convertibility, and a syntactic convertibility;
and although, in some cases, the line of demarcation is not easily drawn
between them, the distinction is intelligible and convenient.

s. 423. _Etymological convertibility._--The words _then_ and _than_, now
adverbs or conjunctions, were once cases: in other words, they have been
converted from one part of speech to another. Or, they may even be said to
be cases, at the present moment; although only in an historical point of
view. For the practice of language, they are not only adverbs or
conjunctions, but they are adverbs or conjunctions exclusively.

s. 424. _Syntactic convertibility._--The combination _to err_, is at this
moment an infinitive verb. Nevertheless it can be used as the equivalent to
the substantive _error_.

_To err is human_ = _error is human_. Now this is an instance of syntactic
conversion. Of the two meanings, there is no doubt as to which is the
primary one; which primary meaning is part and parcel of the language at
this moment.

The infinitive, when used as a substantive, can be used in a singular form
only.

_To err_ = _error_; but we have no such form as _to errs_ = _errors_. Nor
is it wanted. The infinitive, in a substantival sense, always conveys a
general statement, so that even when singular, it has a plural power; just
as _man is mortal_ = _men are mortal_.

s. 425. _The adjective used as a substantive._--Of these, we have examples
in expressions like the _blacks of Africa_--_the bitters and sweets of
life_-